1. Field of the Invention
This invention relates to an illumination device. More particularly, the present invention relates to a plane illumination device for illuminating display panels of liquid crystal television, etc. from the back.
2. Related Background Art
In a liquid crystal display panel, display is effected by discriminating electrooptical modulation of liquid crystals by means of reflected light from an irregular reflection plate placed at the back of the display panel, or otherwise by switching the liquid crystal to control light introduced from the back of the display panel. In general, the former panel is called a reflection-type liquid crystal and the latter panel is called a transmission-type liquid crystal panel. While these two kinds of liquid crystal panels have respective advantages, the transmission-type liquid crystal panel is particularly suitable for use as a display panel for office equipment.
Conventional illumination devices used for transmission-type liquid crystal panels employ a plurality of fluorescent lamps and a light-diffusing screen. However, these illumination devices have drawbacks in that they consume a large amount of electricity when used for display panels of large area (e.g. diagonal size of 12 inches) because of required larger number of fluorescent lamps for illumination and the resulting non-uniform light for illumination. Therefore, plane illumination devices as shown in FIG. 1 are currently in use. In FIG. 1, rays 151 emitted from a light source 11 reflect totaly on a curved reflection plane 12 of a transparent block component 10 and again on the reflection plane portions 13 of a minute-prism array. The reflected rays 152 are converted into diffused rays 153. The diffused rays 153 can illuminate a liquid crystal panel 17 from the back.
This plane illumination device, however, have the following drawbacks. The utilization factor of the rays supplied from the light source is low; a large amount of elecrtricity is consumed for illuminating the liquid crystal to give a sufficient brightness; and the rays for the illumination is inferior in directivity. Accordingly, the net light amount for illuminating the portions of the liquid crystal to be observed becomes inefficient.